Old English hær "hair, a hair," from Proto-Germanic *khæran (cf. Old Saxon, Old Norse, Old High German har, Old Frisian her, Dutch and German haar "hair"), perhaps from PIE *ghers- "to stand out, to bristle, rise to a point" (cf. Lithuanian serys "bristle;" see horror).

Spelling influenced by Old Norse har and Old English haire "haircloth," from Old French haire, from Frankish *harja or some other Germanic source (see above). To let one's hair down "become familiar" is first recorded 1850. Phrase hair of the dog that bit you (1540s), homeopathic remedy, is in Pliny.

One of the fine strands that grow from the skin of mammals, usually providing insulation against the cold. Modified hairs sometimes serve as protective defenses, as in the quills of a porcupine or hedgehog, or as tactile organs, as in the whiskers (called vibrissae) of many nocturnal mammals. Hair filaments are a modification of the epidermis of the skin and are composed primarily of keratin. Hair also contains melanin, which determines hair color.

A slender growth resembling a mammalian hair, found on insects and other animals.

A fine, threadlike growth from the epidermis of plants. See more at trichome.